1. Field of the Invention
The present invention relates to an apparatus for post-processing discharged sheets.
2. Related Background Art
There is already known an apparatus for storing on trays the sheets discharged from a recording apparatus such as a copying machine or a printer on trays, by lowering trays stepwise when a predetermined number of discharged sheets are stacked on a tray, and interrupting the recording operation when the number of stacked sheets reaches an upper limit.
However, a sheet folding unit is often connected to such recording apparatus as mentioned above, and it is difficult, in such case, to ensure satisfactory sheet stacking, since the height of sheets stacked on a tray becomes different depending on whether said sheets are folded in said folding unit or not folded and the detection of the upper limit of the stacked sheet is difficult.
Also at the start of a copying job in such apparatus, satisfactory sheet stacking cannot be expected if the sheets obtained in a preceding copying job are removed from the tray, since the tray has been considerably lowered. In order to eliminate such difficulty, the apparatus can be so constructed to enable the copying job after a pause corresponding to the maximum time required for the tray to return to a reference position for sheet stacking, but such structure may deteriorate the throughput of the apparatus since, if the sheets obtained in the preceding copying job remain on the tray, said returning time is shorter as the upper level of the stacked sheets is close to said reference position.
Also folded sheets or large-sized sheets are often not stacked satisfactorily due to the rigidity of sheets, if they are discharged in the same manner as the unfolded sheets or small-sized sheets.
Also in such conventional apparatus, the processing conditions such as the sheet discharge speed to the tray, initial position of tray, lowering method of tray etc. are predetermined and not adjustable, so that the stacking performance and the matching with the preceding unit are often not satisfied, eventually resulting in dropping or jamming of sheets.